Pipe A alone can fill a tank in 70 minutes. When pipes A and B are together, they can fill the tank in 21 minutes. If pipe B alone fills the tank for 40 minutes, how many litres of water will be wasted? Given that the capacity of the tank is 150 litres.

45 litres
50 litres
80 litres
35 litres

Correct answer: 50 litres

Solution

A fills $1/70$ of the tank per minute and A+B fill $1/21$ per minute. So B's rate is $1/21-1/70=1/30$ tank per minute. In 40 minutes, B fills $40/30=4/3$ of the tank, i.e. 200 litres, so 50 litres are wasted beyond the 150-litre capacity.

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